t was recently pointed out that anti-de Sitter(AdS) spacetime is unstable against gravitational collapse. The pertubation in AdS does not simply decay away and can be reflected by AdS boundary to nonlinearly interact with one another. Confinement would play an important role in the nonlinear instability. For instance, how does the gravitational collapse occur by an effective confinement? In this presentation, I would like to show our results in an asymptotic flat spacetime and discuss open problems in this field after making a brief review.